German patent publication 3,303,350 discloses a control arrangement for the drive power of a motor vehicle wherein the drive power is adjusted by a control of the charging pressure of a turbocharger. This adjustment is in addition to the adjustment via the mechanical throttle flap actuable by the driver. For this purpose, a desired charging pressure is determined via a closed-loop control circuit on the basis of the throttle flap position and the engine speed and is compared to the actual charging pressure. The actual charging pressure is adjusted via a controller by actuating a bypass valve across the turbine of the charger. Also, a precontrol of the charging pressure is provided which adjusts the charging pressure at least in dependence upon the above-mentioned operating variables while bypassing the controller. Desired value input and precontrol values are dimensioned to obtain high power, low fuel consumption and favorable exhaust-gas values.
An internal combustion engine equipped with a turbocharger or a mechanical charger generates a maximum torque at a pregiven rpm when the throttle flap is fully opened and the charging pressure is at a maximum because this results in a maximum cylinder charge. If a lesser torque is required from the engine, then the cylinder charge must be reduced in the same manner as for an induction engine. Stated otherwise, the pressure in the intake pipe must be reduced, that is, the pressure downstream of the throttle flap in the intake system. In known control arrangements, the charging pressure (that is, the pressure upstream of the throttle flap in the intake system) is adjusted in dependence upon the position of the throttle flap pregiven by the driver or the accelerator pedal position proportional to this throttle flap position. For this reason, a reduction of the torque output of the engine takes place primarily because of a throttling of the pressure in the intake pipe as a consequence of the throttle flap being pivoted in the closed direction by the driver.
This, however, leads to the situation that the efficiency of the engine is reduced because of the throttling of the inducted air flow by adjusting the throttle flap. Accordingly, the engine operates at an unfavorable efficiency.